The present invention relates generally to maintaining the power of a DC motor constant when the armature thereof is changing speed, and particularly to maintaining constant power when the field of the motor becomes saturated.
In the context of the following description, the word "saturation" refers to the condition in which the iron structure of a motor field is fully, physically saturated with magnetic flux, as opposed to the intermediate condition that occurs between full saturation of the iron and the condition that produces linear increases in field flux in response to linear increases in field current.
In reducing the gauge of material in a rolling mill, for example, it is necessary that a proper and constant tension be maintained on the material entering and leaving the mill. If proper tension into the mill is not maintained, the material tends to wander from the centerline of the mill which, in turn, can cause buckling and damage to the edges of the wandering material. If the tension on the material leaving the mill is not properly maintained, as the material winds on a rewind or take-up reel, the material tends to wind on the reel in a telescoping fashion, i.e., each successive wrap of the material on the rewind reel displaces itself laterally from the previous wrap.
As a coil of material winds or unwinds, the diameter and radius of the coil changes. The changes in diameter, of course, directly result in changes in the rotational speed of the coil, with constant linear speed of the material leaving or entering the coil of material. In addition, a decreasing radius causes an increasing force and tension on the material being pulled from the coil because the mechanical advantage or lever offered to the mill by the coil radius in the process of pulling the material from the coil is decreasing, thereby providing a motor driving the coil with the increasing ability to resist the pull of the mill. Because of the changing speed and tension experienced in unwind and rewind processes, coils of material are usually driven by DC motors controlled in a manner that maintains proper tension on the material. One system providing such control in a rolling process involves the development of a signal representing the rate of travel of the material through the mill and the voltage measured across the armature of the motor, the armature being mechanically connected to the reel and coil. As the rate of rotation of the coil increases (in an unwinding process), the rate of rotation of the armature increases such that the voltage across the armature increases. The signal developed from armature voltage and the rate of material travel is employed to counter the rising voltage in the armature by reducing the amount of current supplied to the field and thus the magnetic flux of the field. The net effect of this counteraction is to keep the armature voltage at a constant value but proportional to the speed of the sheet. (The current supplied to the armature of the motor is held constant by an independent power supply). In addition, because of the decreasing current and flux in the field, the torque of motor is decreasing. In this manner, the torque applied to the shaft of the motor varies inversely with the speed of the motor and therefore the diameter of the coil so that the force applied to the coil of material, as it is pulled from the coil, is maintained constant. Since both the current supplied to the armature and its voltage are constant, the power developed by the armature is held constant.
Another arrangement for controlling force on an unwind or rewind reel, via control of a DC motor, is disclosed in U.S. Pat. No. 3,749,988 to Pittner. Pittner is concerned with extending the range of motor speed associated with coils of material as they approach minimum diameters in the process of paying off the material of a coil, or at the beginning of the take-up process, when diameter of the coil is quite small. On such occasions the speed of the motor approaches the upper limit of its speed range in the process of maintaining proper tension on the material being paid off or taken-up. Increases in motor speed above this limit endanger the motor since motors have design limitations. The speed of the motor in the Pittner disclosure is controlled by measuring the diameter of the coil of material being driven by the motor and adjusting flux in the motor field as a function of the latest measurement of coil diameter during the period of time the diameter is greater than a small, finite diameter; during this time armature current is maintained constant. When, however, the latest measurement of diameter is equal to or below the finite diameter, field flux is held constant and the operator reduces line speed so that the motor will not exceed its highest rated voltage and speed. When line speed is reduced, armature current is readjusted downwardly to reflect such lower speed.